The invention relates to a method for levelling out tension variation of an optical fibre when the fibre is wound on or off a reel, the basic tension of the fibre being provided by means of a dancer. The invention also relates to an arrangement for levelling out tension variation of an optical fibre when the fibre is wound on or off a reel, the basic tension of the fibre being provided by means of a dancer.
In connection with treatment of optical fibres, it has been found that when an optical fibre is wound on or off a reel, it is disadvantageous that the tension of the fibre varies. Too high a tension may have a detrimental effect--either temporary or permanent--on the optical properties of the fibre. Too low a tension, in turn, may lead to formation of "loose loops" when the fibre is wound on a reel; such loops cause attenuation steps in the measurement results.
Tension variations are also disadvantageous in the treatment processes of the fibre. When a fibre is wound off a reel for coating, for instance, tension variation may make the fibre vibrate, which results in unequal wall thickness of the fibre coating.
When a fibre is wound off a reel, tension variations occur, for example, because of uneven winding rate, which may result from eccentricity of the reel or unevenly performed winding of the fibre on the reel. Tension variations also occur when a fibre is wound on a reel on account of an uneven reel, eccentricity, clearings, etc. Eccentricities of guide wheels on the fibre path, clearings, friction variations of bearings, etc., also cause tension variations in the fibre. In addition, tension variations are caused by air flows and other ambient disturbances which make the fibre vibrate. Resonance frequencies must also be borne in mind. Yet another source of tension variations is forces exerted on the fibre by static electricity.
Several solutions have been provided for controlling tension variations. As a first example can be mentioned balanced lever dancers, which tend to level out tension. The disadvantage of this solution is its response speed, which is limited by the law a=F/m. The drawbacks of the solution thus result from the inertial forces of mass.
A second example of known solutions is lever dancers operated by a spring, compressed air or the like. In such solutions, the proportion of mass to force may be lower than in the previous example, wherefore a=F/m is more advantageous and the solution is faster than that of the first example. However, the velocity of even this known solution is not always sufficient for levelling out rapid tension variations.
A third example of known solutions is linear dancers. They have the same drawbacks as the examples described above.
A fourth example is tension-controlled hauling devices, such as capstans. These solutions have the same drawback as the examples described above, i.e. their velocity is not sufficient. In addition, such a construction is expensive--extremely expensive if the device is to be very fast.